Seymour Duncan Convertible 100 - Relay Switching Issue

[Gahoo]

Working on a Seymour Duncan Convertible 100 combo that works fine except for the relay switching circuit.




The switch works as expected and so does the jack. The red and green LEDs light accordingly.

Initially the relay would click intermittently, not at all, or sometimes after a delay of a couple seconds. When the relay did activate, the corresponding LED would light a bit brighter. I replaced the relay, no luck, same behavior.

I replaced the capacitor across the coil, thinking that was the cause of the time delay. No dice.

Replaced the 2N4401 transistor on the green LED line, no change.

Also worth noting that the amp I have in front of me has the R18/R19 node connected to the base of that 2N4401, it's not drawn that way on the schematic and I'm not sure why..

Can anyone see something I'm missing?

Thanks!

[mozwell]

The junction of the resistors should go to the base of the transistor. Must be a drawing error.

Attached is the sch for the amp, and modules
The relay gets its power from H, the output of a 317 regulator
Check the voltage at H, which should be approx 9.8VDC although the sch says 10.6VDC
When the red LED is on check the DC voltage wrt 0V at both sides of R30 and across K1 itself

What is the relay part number, what coil voltage is it

I am guessing the relay doesn't have enough voltage across its coil to turn it on correctly.
If it is a 9V relay, then with the led, and the 33R resistor, we may only get 8V across it.

H also powers a LM380 and a 5534 op amp, the LM380 can be powered from up to 22VDC, so by changing R39 in the power supply, we can raise the voltage at H a little. Adding 220R is series with the existing R39 1K5 will bump the volts at H up by 1.5V or so

Good luck

[Gahoo]

Thanks mozwell!

The relay I'm using is a DS2E-S-DC6V (so a 6V coil)

With the Red LED illuminated:

Getting 10.8VDC at H
On the other side of R30 I'm getting 10.7VDC
Across the coil I get .64VDC

So I check the voltage into the LED: 10.14VDC
Out of it, 6mV since it's grounded by the footswitch.

[mozwell]

If the red led is always on, just brighter / dimmer, then my request for measurements may not be the best.
With the foot switch open, you should not get very much volts across the relay coil. With the foot switch closed, you should get more than 6V across the relay coil.
Please make these measurements & post results.
We want to find out if the relay coil is getting the correct voltage, then we can work out why this isnt switching.

Are you sure you replaced the capacitor C32 with the correct polarity

[Gahoo]

The Red LED isn't always on, it actually switches as expected. When it is on it's just a touch dimmer until the relay clicks (if it clicks at all).

With the footswitch open I'm getting .26mV across the coil, with it closed I get .76V

*Also worth adding that I've applied 6v across the coil briefly to verify that it works as expected when it gets the proper voltage.

[Gahoo]

Replaced the red LED and now the relay works as expected! The LED stays on constantly but is significantly brighter in the switch closed position.

That would've been about the last component I was suspecting, especially given that it was still lighting (albeit dimly). Anyone have any idea why replacing it would have solved the relay issue? I'm guessing the LED I used as a replacement has a lower forward voltage?

[52 Bill]

Replaced the red LED and now the relay works as expected! The LED stays on constantly but is significantly brighter in the switch closed position.

The forward voltage of the led shouldn't make a difference in the circuit, it may have just been a bad led. With 57K in series to ground, the red led should turn off when the switch is turned off.

The current through the red led is limited by the resistor R30 and the resistance of the relay coil. When you changed the relay did you use one with a similar coil resistance value? Or did you return the original relay to the circuit?

[Gahoo]

I think with the base of that transistor connected to the junction of R18 and R19 it's actually not just 57k in series to ground.

It didn't even occur to me that the resistance of the coil plays a part in the current limiting of the led, I guess I figured it would be so low it'd be inconsequential
The relay I had removed had an open coil so I wasn't able to test it's resistance, the replacement has a resistance of 180 Ohms.

[mozwell]

Glad you got it working